This invention is directed to a membrane and diaphragm-free process for reducing the concentration of metal(s) in aqueous solutions of plating wastes, particularly those involving metal plating operations using copper, cadmium, zinc, nickel and related metals. According to this invention, the metal plating waste aqueous solution containing the metal(s) desired to be reduced in concentration is passed into contact with a plurality of anodes having openings therein to permit the flow of waste water therethrough and a plurality of reticulated cathodes in an electrolytic cell wherein each reticulated cathode is comprised of an organic polymer foam substrate containing an electroless then electrolytic deposit of metal or alloy on and within the pores of said foam. In accordance with one embodiment of the invention, concurrent metal removal and cyanide destruction are conducted using a combination of metallized organic polymer foam reticulate electrodes for both anodes and cathodes in a recirculating (recycle) mode. The process of this invention permits removal of a significant portion of metal and facilitates metal recovery from plating waste waters in a form whereby the recovered metal can be recycled for subsequent use.
There is considerable and growing concern over pollution of the nations' waterways with various contaminants such as the heavy metals, e.g., copper, nickel, zinc, mercury, cadmium, etc., and such nonmetallic contaminants as cyanide as well as complexes of the foregoing metals with cyanide, etc. Many of these pollutants enter the nations' waters from industrial sources, such as metal finishing or plating plants and from mining sources. Environmental legislation and regulations, on the federal, state and local government levels, have set forth maximum allowable concentrations of these contaminants which may be discharged into public waters. A clear and present need exists for an economical yet effective process for treating such waste waters to permit removal of a substantial portion of contaminants, especially with respect to platers' rinse water, which customarily contains one or more of the aforementioned metals as such or in combination with cyanide.
Aqueous plating wastes comprise various aqueous solutions containing silver, copper, cadmium, mercury, zinc, nickel and other metals with or without cyanide. These plating wastes must be treated prior to discharge.
Presently the most common method for removing or reducing the concentration of metals from aqueous plating waste solutions prior to their disposal or discharge is by chemical treatment. Since different metals precipitate and/or are reacted under different conditions, e.g., at different pH levels, utilization of a process for treatment for various plating waste streams is difficult. Furthermore, most chemical treatment methods remove the metal by reaction and subsequent precipitation which causes a metal sludge which must be disposed of. Sometimes this makes for environmental complications. Therefore, it would be desirable to provide a metal removal process capable of reducing significantly the concentration of metal found in platers' rinse waters which is capable of achieving the low tolerable permitted concentrations for discharge or disposal while at the same time recovering the metal in a pure state and not producing any toxic sludges. Also economic considerations call for a metal treatment process wherein the platers' rinse waters can be treated preferably by a one-pass procedure, viz., one which does not require recirculation of the platers' rinse water through the equipment employed to conduct the metal removal process.
The present invention provides a process for electrolytically reducing the concentration of the desired metal(s) present in the platers' rinse water enabling a single pass or a recycling system through the equipment which contains a plurality of anodes and cathodes, with at least each cathode being comprised of an open cell (interconnected cell) organic polymer foam substrate upon which have sequentially been deposited an electroless then an electrolytic metal layer appropriate to cause cathodic deposition of the metal sought to be removed or whose concentration is sought to be significantly reduced so as to enable the effluent to be discharged without requiring further treatment.
Electrolytic attempts to remove metals, such as those described above, from platers' aqueous rinse waters are known. For example, U.S. Pat. No. 3,954,594 to Howard L. Recht is directed to a two-compartment electrolytic cell wherein heavy metals are deposited upon electrically conductive particles located in a cathode compartment. Separate from the cathode compartment there is an anode compartment also containing a bed of electrically conductive particles which is employed to neutralize, viz., by oxidation, cyanide also contained in the waste water. Upon passage of a direct electric current between the two compartments, heavy metal is deposited upon the particles in the cathode compartment and the cyanide is oxidized in the anode compartment.
U.S. Pat. No. 4,197,181 issued to Charles Portal et al is directed to a stationary particulate bed dual electrode utilizing a stationary bed of carbonaceous particles to remove metals from aqueous rinse waters. This Portal et al electrode also incorporates a foam filter element, e.g., low density polyurethane foam, to filter particulate material present in the electrolyte solution. At column 5, lines 61 to 62, it is stated that fibrous mats of graphite carbon or carbon felt may be substituted for the carbonaceous particles. The carbonaceous particles, e.g., coke, are held in place in the compartment by an interface liner, e.g., synthetic filter cloth. The Portal et al stationary particulate bed dual electrode employs two stationary particulate bed electrode assemblies adapted for mating to one another in an electrically connected relationship. When so mated, the electrode assemblies create an internal cavity for the introduction of metalic ion containing solutions from which the metal values are to be extracted.
U.S. Pat. No. 4,226,685 issued to Charles Portal et al (a continuation-in-part application of U.S. Pat. No. 4,197,181) is directed to a method of treating plating wastes containing at least one heavy metal and (optionally) cyanide ions such that ionic contaminants, e.g., copper, are reduced to concentrations of 33 parts per million (one pass) and 15 parts per million (two passes) compared with the initial concentration (prior to contact with said electrodes) of 150 parts per million (as cupric sulfate). The deposited copper is deposited on the particulate stationary bed of carbonaceous particles, e.g., charcoal particles, and the beds are disassembled so the metal-plated carbonaceous particles can be removed and replaced. At column 6, lines 19 to 22, it is stated that the metal loaded cathodes may be treated by conventional purifying techniques to obtain useful metals and alloys or plating solutions.
An article entitled "Characterization of Reticulate, Three-Dimensional Electrodes" by A. Tentorio et al published in the JOURNAL OF APPLIED ELECTROCHEMISTRY 8 (1978) pages 195 to 205 is directed to electrodes prepared from open cell polyurethane foam upon which there is deposited copper, first by electroless deposition followed by electrolytic deposition. In FIG. 3 of the Tentorio et al article, there is shown an exploded view of an electrolytic cell utilizing such a copper plated polyurethane foam reticulate electrode (cathode) in conjunction with a lead/lead oxide counter-electrode (anode) separated by an ion exchange membrane. The solution to be treated i.e., the copper bearing rinse water as described by the article, is pumped through the catholyte compartment of the cell. A separate anolyte solution of 1MH.sub.2 SO.sub.4 was pumped independent of the catholyte through the anolyte compartment of the cell. At page 205 of the Tentorio et al article, the authors state that the cell of FIG. 3 could operate in a waste water treatment system only with multiple pass electrolysis, and only with the level of concentration of polutant, i.e., copper, after treatment not below tens of parts per million in order not to weigh down excessively the recycle. Thus it seems clear that the authors contemplate the use of such copper plated polyurethane foam reticulate electrodes only in cases where the waste water to be treated is to be subjected only to multiple pass electrolytic treatment through a cell separated by an ion exchange membrane into an anolyte and catholyte chamber and where the desired concentration of polutants in the effluent water from the treatment process is at least counted in terms of tens of parts per million.